Regenerator material for extremely low temperatures and regenerator for extremely low temperatures using the same

ABSTRACT

A cold heat accumulating material for extremely low temperatures which comprises cold heat accumulating granular bodies in which a rate of particles, which are destroyed when a compressive force of 5 MPa is applied thereto by a mechanical strength evaluation die, out of the magnetic cold heat accumulating particles constituting the magnetic cold heat accumulating granular bodies is not than 1 wt. %. In this magnetic cold heat accumulating granular bodies, a rate of magnetic cold heat accumulating particles having more than 1.5 form factor R expressed by L2/4πA, wherein L represents a circumferential length of a projected image of each magnetic cold heat accumulating particle, and A a real of the projected image, is not more than 5%. Such a cold heat accumulating material for extremely low temperatures is capable of providing excellent mechanical properties with respect to mechanical vibration with a high reproducibility. A cold heat accumulator for extremely low temperatures is formed by filling a cold heat accumulating container with a cold heat accumulating material for extremely low temperatures comprising the above-mentioned magnetic cold heat accumulating granular bodies. Such a cold heat accumulator for extremely low temperatures can display excellent performance for a long period of time.

TECHNICAL FIELD

The present invention relates to a regenerator material for extremelylow temperatures for use in refrigerators and such like and aregenerator for extremely low temperatures using the same.

BACKGROUND OF ART

In recent years there have been notable developments in superconductingtechnology, and along with expansion in relevant fields of applicationthe development of compact and high performance refrigerators has becomeessential. Such refrigerators demand light weight, compactness and highefficiency.

For instance, refrigerators with freezing cycles such as the GiffordMacMahon system or the Sterling system have been used in superconductingMRI and cryopump and the like. In addition, high performancerefrigerators are indispensable for magnetic levitation trains. In suchrefrigerators, an operating medium such as compressed He gas flows inone direction through a regenerator filled with regenerator material andsupplies the resulting thermal energy to the regenerator material, andthe expanded operating medium then flows in the opposite direction andreceives thermal energy from the regenerator material. In this process,as the regenerative effect is improved, thermal efficiency of theoperating medium cycle is increased and it becomes possible to achieveeven lower temperatures.

Cu or Pb and the like have conventionally been used as regeneratormaterial in the above-mentioned refrigerators. However, specific heat ofsuch regenerator material becomes noticeably low at extremely lowtemperatures below 20 K and consequently the above-mentionedregenerative effect does not function sufficiently making it difficultto achieve extremely low temperatures.

Therefore, in order to achieve temperatures closer to absolute zero, theuse of magnetic regenerator materials which exhibit substantial specificheat in extremely low temperatures such as Er--Ni type intermetalliccompounds such as Er₃ Ni, ErNi, ErNi₂ (See Japanese Patent Laid-OpenApplication No. Hei 1-310269) or ARh type intermetallic compounds (A:Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb) (See Japanese Patent Laid-OpenApplication No. Sho 51-52378) such as ErRh is recently being considered.

However, during operation of the above-mentioned regenerators, theoperating medium such as He gas passes at high pressure and high speedthrough gaps in the regenerator material with which the regenerator isfilled and consequently the flow direction of the operating mediumchanges at frequent intervals. As a result, the regenerator material issubject to a variety of forces such as mechanical vibration. Stress isalso applied when filling the regenerator with the material

Though the regenerator material is subject to the various forces,magnetic regenerator material of the intermetallic compounds describedabove such as Er₃ Ni or ErRh is generally brittle and consequently isprone to pulverization as a result of mechanical vibration duringoperation or pressure during filling or such like. The particlesgenerated by this pulverization influence harmfully the performance ofthe regenerator, such as obstructing the gas seal. Moreover, there isalso the problem that the degree of deterioration in the performance ofthe regenerator when using a magnetic regenerator material of theintermetallic compounds as described above varies widely depending themanufactured batches of magnetic regenerator material and the like.

It is therefore the object of the present invention to provide aregenerator material which have excellent mechanical properties formechanical vibration and filling stress and such like with a highreproducibility, a regenerator which have excellent refrigeratingperformance in extremely low temperature over a long period of time witha high reproducibility by using such a regenerator material, and arefrigerator using such a regenerator for extremely low temperatures.

DISCLOSURE OF THE INVENTION

Having considered various means for achieving the objectives describedabove, the present inventors have discovered that the mechanicalstrength of magnetic regenerator material particles of intermetalliccompounds and such like containing rare earth elements is highlydependent on the precipitation volume, the precipitation situation, theform and such like of rare earth carbides and rare earth oxides, whichexist in the grain boundary. The precipitation volume and precipitationsituation and such like of these rare earth cabides and rare earthoxides are complexly related to the amount of carbon and oxideimpurities, atmosphere in the rapid solidification process, coolingvelocity, melt temperature and such like, and therefore they altergreatly depending the manufactured batch of the magnetic regeneratormaterial particles. It was discovered that the mechanical strength ofthe magnetic regenerator particles therefore varies greatly with eachmanufactured batch and that it would be extremely difficult to predictmechanical strength from manufacturing conditions and such like alone.

In order to improve the mechanical reliability of magnetic regeneratorparticles, following detailed consideration of the mechanical propertiesof magnetic regenerator particles, it was learned that mechanicalreliability of magnetic regenerator particles can be estimated byconsidering the mechanical strength of not an individual magneticregenerator particle but an aggregation of magnetic regeneratorparticles, concentration of stress when a force is applied toaggregation of magnetic regenerator particles. With regard to the formof magnetic regenerator particles, it was further discovered that it ispossible to increase the mechanical reliability of magnetic regeneratorparticles by selectively using magnetic regenerator particles with aform having few protrusions. The present invention is based on these newknowledges.

In other words, a first regenerator material for extremely lowtemperatures of the present invention is characterized in that itcomprises aggregation of magnetic regenerator particles, in which a rateof the particles which are fractured is not more than 1 wt. % when acompressive stress of 5 MPa is applied thereto.

A first regenerator for extremely low temperatures of the presentinvention comprises a regenerator container filled with theabove-mentioned first regenerator material for extremely lowtemperatures.

Furthermore, a second regenerator material for extremely lowtemperatures of the present invention is characterized in that itcomprises aggregation of magnetic regenerator particles, in which a rateof the particles satisfying that form factor R is more than 1.5, whereinR is expressed by L² /4πA, L represents a perimeter of a projected imageof the individual regenerator particle and A represents an area of theprojected image, is not more than 5%.

A second regenerator for extremely low temperatures of the presentinvention comprises a regenerator container filled with theabove-mentioned second regenerator material for extremely lowtemperatures.

Moreover, a refrigerator of the present invention includes theabove-mentioned first regenerator for extremely low temperatures or thesecond regenerator for extremely low temperatures.

A regenerator material for extremely low temperatures of the presentinvention consists of magnetic regenerator particles, namely anaggregate of magnetic regenerator particles. For instance, intermetalliccompounds including rare earth elements expressed by RM_(Z) (Rrepresents at least one rare earth element chosen from Y, La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; M represents at least onemetallic element chosen from Ni, Co, Cu, Ag, Al and Ru; z represents anumber between 0.001.sup.˜ 9.0) or intermetallic compounds includingrare earth elements expressed by ARh (A represents at least one rareearth element chosen from Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb) areappropriate as the magnetic regenerator material in the presentinvention.

When the magnetic regenerator particles described above have almostspherical form and are uniform in size, they can smooth out the flow ofthe gas. Consequently, not less than 70 wt. % of the whole magneticregenerator particles can suitably be constituted with magneticregenerator particles each having a shape such that the ratio of themajor diameter to the minor diameter (aspect ratio) is not greater than5, and with a diameter of 0.01.sup.˜ 3.0 mm.

When the magnetic regenerator particle aspect ratio exceeds 5, itbecomes difficult to fill to make gaps uniform. Consequently when suchparticles exceed 30 wt. % of the whole magnetic regenerator particles,the regenerator performance and the like may deteriorate. The aspectratio should preferably be not more than 3 and ideally not more than 2.Furthermore, the rate of magnetic regenerator particles with a particleaspect ratio of not more than 5 should preferably be not less than 80wt. % and ideally not less than 90 wt. %.

Moreover, when the diameter of the magnetic regenerator particles isless than 0.01 mm, the packing density becomes too much, thereby thepressure loss of working medium such as helium is likely to increase. Onthe other hand, when the particle size of the magnetic regeneratorparticles is more than 3.0 mm, the area of heat transfer surface betweenthe magnetic regenerator particles and the working medium becomes small,thereby heat transfer efficiency deteriorates. Accordingly, when thepercentage of such particles is more than 30% by weight of the magneticregenerator particles, the regenerator performance etc. is likely todeteriorate. The particle size is preferably in a range of 0.05.sup.˜2.0 mm, more preferably in a range of 0.1.sup.˜ 0.5 mm. The percentageof the particles having a diameter ranging 0.01.sup.˜ 3.0 mm in thewhole magnetic regenerator particles is preferably not less than 80% byweight, more preferably not less than 90% by weight.

A regenerator material for extremely low temperatures of the presentinvention comprises magnetic regenerator particles in which the rate ofparticles which are fractured when a compressive stress of 5 MPa isapplied to an aggregate of magnetic regenerator particles with theabove-mentioned form is not more than 1 wt. %. As described above, thepresent invention considers the mechanical strength of an aggregate ofmagnetic regenerator particles in which the mechanical strength of eachregenerator particle for extremely low temperatures is complexly relatedto the volume of carbon and oxide impurities, atmosphere during therapid solidification process, cooling velocity, melt temperature andsuch like, and wherein a complex concentration of stress occurs whenstress is applied to an aggregate of these particles. By measuring therate of particles fractured when a compressive stress of 5 MPa isapplied to such aggregates of magnetic regenerator particles, it ispossible to evaluate the reliability of the magnetic regeneratorparticles with respect to mechanical strength.

In other words, when the rate of particles fractured when a compressivestress of 5 MPa is applied to an aggregate of magnetic regeneratorparticles is not more than 1 wt. %, hardly any magnetic regeneratorparticles are pulverized as a result of mechanical vibration during anoperation of refrigerator or by stress and such like when filling theregenerator container with these particles, even if the manufacturingbatches and manufacturing conditions are different. Therefore, theproblems such as obstruction of gas seals in refrigerators and the likecan be prevented by using magnetic regenerator particles with thesemechanical properties. The reliability cannot be evaluated, since mostmagnetic regenerator particles, irrespective of their internalmorphology, are not fractured by the application of a compressive stressof less than 5 MPa.

The above-mentioned reliability evaluation of magnetic regeneratorparticles is carried out as follows. First, a fixed amount of magneticregenerator particles is extracted randomly from each manufacturingbatch which comply with a specified aspect ratio, particle size and suchlike. Second, as FIG. 1 shows, the extracted magnetic regeneratorparticles 1 are filled within a die 2 for the mechanical strengthevaluation and a stress of 5 MPa is applied thereto. The stress needs tobe increased gradually; for instance, crosshead speed in these tests isroughly 0.1 mm/min. Furthermore, the die 2 material is die steel andsuch like. After stress has been applied, fractured magnetic regeneratorparticles are sorted by sieving and shape separation, and thereliability of the aggregate of magnetic regenerator particles isevaluated by measuring the weight of the fractured particles. Anextraction of around 1 g of magnetic regenerator particles from eachmanufacturing batch is sufficient.

The rate of particles fractured when a compressive stress of 5 MPa isapplied to magnetic regenerator particles should preferably be not morethan 0.1 wt. % and ideally not more than 0.01 wt. %. In addition, for areliability evaluation of magnetic regenerator particles, the rate ofparticles fractured when a compressive stress of 10 MPa is appliedthereto should preferably be not more than 1 wt. % and should ideallysatisfy the same conditions when a compressive stress of 20 MPa isapplied.

A regenerator material for extremely low temperatures of the presentinvention can basically prevent the generation of pulverization ofparticles by satisfying the above-mentioned mechanical strength ofaggregates of magnetic regenerator particles when a compressive stressis applied thereto, and mechanical reliability can be further improvedin order to be capable of preventing more effectively the chipping andsuch like by the use of magnetic regenerator particles with a form asdescribed below.

In other words, regenerator particles should preferably have a sphericalform as explained above and when this form is more precisely sphericaland the size of the particles is more uniform, the flow of the gas canbe smoothed out and extreme stress concentration occurring when acompressive stress is applied to these particles can be restricted.Mechanical vibration during refrigerator operation or stress appliedwhen the regenerator is filled with regenerator material are conceivableas the above-mentioned compressive stress. The stress is most likely toconcentrate when particles with a less spherical form are subjected to acompressive stress.

Conventionally, only the ratio of the major diameter to the minordiameter (i.e. the aspect ratio) has been used when evaluating thespherical form of magnetic regenerator particles (for instance, seeJapanese Patent Laid-Open Application No. Hei 3-174486). However, theaspect ratio tends to be a lower value when the roundness of an ellipseis evaluated although it is valid as a parameter for evaluating thewhole particle form, even if there are protrusions on the particlesurface for example these protrusions have little influence on theaspect ratio.

When the magnetic regenerator particles used as regenerator material forextremely low temperatures comprise particles with complex surface formssuch as protrusions, stress concentrate on the protrusions and such likewhen a compressive stress is applied, and the mechanical strength of themagnetic regenerator particles is thereby adversely affected. Thereforein the present invention, a rate of regenerator particles satisfyingthat form factor R is greater than 1.5, wherein R is expressed by L²/4πA, L represents a perimeter of a projected image of the individualmagnetic regenerator particles and A represents an area of the projectedimage, is preferably not more than 5%.

As FIG. 2 shows, when protrusions are present on the particle surface,even a particle with a highly spherical form will have a high formfactor R value (high partial shape irregularity). Furthermore, as FIG. 3shows, a particle with a comparatively smooth surface will have a lowform factor R value even if its form is rather unspherical. In contrast,the aspect ratio described above tends to be a lower value for particlessuch as that shown in FIG. 3 (aspect ratio=b/a) and a higher value forparticles with surface protrusions and the like such as shown in FIG. 2.

In other words, a low form factor R indicates that the particle surfaceis comparatively smooth (low partial shape irregularity) and R is aneffective parameter for evaluating partial form irregularity ofparticles. Therefore, by using particles with a low form factor R it ispossible to achieve improvements in the mechanical strength of magneticregenerator particles. In fact, even particles whose aspect ratioexceeds 5 do not adversely affect the mechanical strength of magneticregenerator particles substantially provided that the particle surfaceis smooth. On the other hand, when particles with the projections andsuch like have high partial form irregularity and their form factor Rexceeds 1.5, the projections are liable to chip and consequently suchparticles have poor mechanical strength. Therefore, when the rate ofsuch particles with high partial form irregularity exceeds 5%, themechanical strength of the magnetic regenerator particles is adverselyaffected.

Based on the reasons described above, the rate of particles with a formfactor R exceeding 1.5 should preferably not be more than 5%, morepreferably not more than 2% and ideally not more than 1%. Furthermore,the rate of particles with a form factor R exceeding 1.3 shouldpreferably not be more than 15%, more preferably not more than 10% andideally not more than 5%. However, since the aspect ratio is importantfor evaluating the degree of sphericity, having satisfied form factor Rprovisions, not less than 70 wt. % of the magnetic regenerator particlesshould preferably have an aspect ratio of not more than 5 as describedabove.

The manufacturing method of magnetic regenerator particles describedabove is by no means restricted and a variety of manufacturing methodscan be employed. For instance, melt of a designated composition can berapidly solidified using methods such as centrifugal atomization, gasatomization and rotational electrode method. In addition, magneticregenerator particles in which a rate of particles satisfying that formfactor R is greater than 1.5 is not more than 5%, can be obtained by forinstance optimizing manufacturing conditions and carrying out shapeseparation such as inclined vibrating plate method.

A regenerator for extremely low temperatures of the present inventionuses magnetic regenerator particles having mechanical properties asdescribed above, namely magnetic regenerator particles with a rate ofparticles fractured when a compressive stress of 5 MPa is applied of notmore than 1 wt. %. Moreover a regenerator for extremely low temperaturesof the present invention can be composed of magnetic regeneratorparticles with a rate of particles satisfying that form factor R isgreater than 1.5 of not more than 5%. A regenerator for extremely lowtemperatures wherein a regenerator has been filled with magneticregenerator particles satisfying both mechanical properties and form isespecially preferable.

Since magnetic regenerator particles used in a regenerator for extremelylow temperatures of the present invention contain hardly any magneticregenerator particles which are pulverized as a result of mechanicalvibration during a refrigerator operation or compressive stress whenfilling the container of a regenerator, and such like, obstruction ofgas seals in refrigerators and such like can be prevented. Therefore, aregenerator for extremely low temperatures capable of steadilymaintaining refrigerating performance over a long period of time andmoreover a refrigerator capable of steadily maintaining refrigeratingperformance over a long period of time can be obtained with highreproducibility.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional drawing depicting an example of a die usedfor mechanical strength evaluation in order to evaluate the reliabilityof magnetic regenerator particles of the present invention.

FIG. 2 is a schematic drawing showing a relation between an example formof a magnetic regenerator particle and a parameter to evaluate degree ofsphericity.

FIG. 3 is a schematic drawing showing a relation between another exampleform of a magnetic regenerator particle and a parameter to evaluatedegree of sphericity.

FIG. 4 is a drawing depicting a configuration of a GM refrigeratormanufactured in an embodiment of the present invention.

MODE FOR EMBODYING THE INVENTION

The preferred embodiments of the present invention will next beexplained.

Embodiment 1

First, an Er₃ Ni mother alloy was prepared by high frequency fusion.This Er₃ Ni mother alloy was melted at approximately 1373 K and the meltthereby obtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidified. The particlesobtained were sieved and classified according to form and 1 kg ofspherical particles with diameters of between 0.2.sup.˜ 0.3 mm wasselected. Particles with an aspect ratio of not more than 5 constitutednot less than 90 wt. % of all the particles in these particles. Thisprocess was carried out repeatedly and 10 batches of spherical Er₃ Niparticles were obtained.

Next, 1 g of particles was randomly extracted from each of the tenbatches of spherical Er₃ Ni particles. These extracted particles wereeach filled within a die 2 for mechanical strength evaluation shown inFIG. 1 and a compressive stress of 5 MPa (crosshead speed=0.1 mm/min)was applied using an Instron-type testing machine. Following the test,all particles were sieved and classified according to form and theweight of the fractured spherical Er₃ Ni particles was measured. Thebatch in which the fractured particle rate was 0.004 wt. % was selectedas magnetic regenerator particles for this embodiment. When the formfactor R of these magnetic regenerator particles in this batch wasevaluated by image analysis, the rate of particles having a form factorR of more than 1.5 was not more than 5%.

Magnetic regenerator spherical particles comprising Er₃ Ni selected inthe manner described above were filled in a regenerator container at apacking factor of 70% to construct a regenerator for extremely lowtemperatures. A two-stage GM refrigerator, which is shown schematicallyin FIG. 4, was constructed using this regenerator for extremely lowtemperatures and refrigerator testing was carried out. Test resultsshowed an initial refrigeration capacity of 320 mW was obtained at 4.2 Kand stable refrigeration capacity was obtained throughout 5000 hours ofcontinuous operation.

The two-stage GM refrigerator 10 shown in FIG. 4 has a vacuum chamber 13provided with a large-diameter first cylinder 11 and a small-diametersecond cylinder 12 which is cocentrically connected thereto. A firstregenerator 14 can reciprocate in the first cylinder 11 and a secondregenerator 15 can reciprocate in the second cylinder 12. Seal rings 16and 17 are provided respectively between the first cylinder 11 and thefirst regenerator 14 and between the second cylinder 12 and the secondregenerator 15.

The first regenerator 14 contains a first regenerator material 18 suchas Cu mesh. The second regenerator 15 is configured according to aregenerator for extremely low temperatures of the present invention andcontains a regenerator material for extremely low temperatures 19 of thepresent invention as a second regenerator material. The firstregenerator 14 and the second regenerator 15 have passages for anoperating medium such as He gas provided in the gaps and such like ofthe first regenerator material 18 and the regenerator material forextremely low temperatures 19 respectively.

A first expansion space 20 is provided between the first regenerator 14and the second regenerator 15. A second expansion space 21 is providedbetween the second regenerator 15 and the cold stage of the secondcylinder 12. A first cooling stage 22 is formed in the lower portion ofthe first expansion space 20 and a second cooling stage 23 at a lowertemperature than the first cooling stage 22 is formed in the lowerportion of the second expansion space 21.

A compressor 24 supplies a high pressure operating medium (e.g. He gas)to the above-mentioned two-stage GM refrigerator 10. The suppliedoperating medium passes through the first regenerator material 18contained in the first regenerator 14 and reaches the first expansionspace 20, then passes through the regenerator material for extremely lowtemperatures 19 (the second regenerator material) contained in thesecond regenerator 15 and reaches the second expansion space 21. In thisprocess, the operating medium cools by supplying thermal energy to bothregenerator materials 18 and 19. Having passed through regeneratormaterials 18 and 19 the operating medium expands and absorbs heat in thefirst and second expansion space 20, 21 and both cooling stages 22 and23 are cooled. The expanded operating medium now flows in reversedirection through both regenerator materials 18 and 19. After receivingthermal energy from the regenerator materials 18 and 19, the operatingmedium is exhaused. This process increases the cooling efficiency of theoperating medium cycle and achieves even lower temperatures, as theregenerator efficiency improves.

Embodiment 2

As in the embodiment 1, 10 batches were produced of spherical Er₃ Niparticles with particle diameters of between 0.2.sup.˜ 0.3 mm of whichparticles with an aspect ratio of not more than 5 constituted not lessthan 90 wt. %. Next, 1 g of particles was randomly extracted from eachof the ten batches of spherical Er₃ Ni particles. These extractedparticles were each filled within the die 2 for mechanical strengthevaluation shown in FIG. 1 and a compressive stress of 5 MPa (crossheadspeed=0.1 mm/min) was applied thereto using an Instron-type testingmachine. Following the test, all the particles were sieved andclassified according to form and the weight of the fractured sphericalEr₃ Ni particles was measured. The rate of fractured particles is shownin Table 1.

The magnetic regenerator spherical particles consisting of Er₃ Ni fromeach of the 10 batches were respectively filled in regeneratorcontainers at a packing factor of 70% and then put in a two-stage GMrefrigerator and refrigerating testing was carried out as in theembodiment 1. The test results are also shown in Table 1.

COMPARATIVE EXAMPLE 1

A batch in which the rate of spherical Er₃ Ni particles fractured when acompressive stress of 5 MPa was applied thereto was 1.3 wt. % wasselected from the 10 batches of spherical Er₃ Ni particles produced inthe embodiment 1. The selected magnetic regenerator spherical particlesof Er₃ Ni were filled in a regenerator at a packing factor of 70%,respectively, and then put in a two-stage GM refrigerator andrefrigerating testing was carried out as in the embodiment 1. The testresults are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                Rate of particles                                                             fractured by  Refrigeration                                                   compressive   capacity (mW)                                                     stress test of  Initial                                                                              After 5000                                   Test No.  5 MPa (wt. %)   Value  hours                                        ______________________________________                                        Embodiment 2                                                                  1         0.001           321    320                                          2         0.007           325    325                                          3         0.840           327    305                                          4         0.014           326    321                                          5         0.001           322    320                                          6         0.110           325    318                                          7         0.021           329    326                                          8         0.008           330    328                                          9         0.045           324    320                                          10        0.216           321    314                                          Comparative                                                                             1.3             320    270                                          Example 1                                                                     ______________________________________                                    

As Table 1 clearly shows, all the regenerators using magneticregenerator particles in which the rate of particles fractured when acompressive stress of 5 MPa was applied was not more than 1 wt. %maintained excellent refrigeration capacity over a long period of time.

COMPARATIVE EXAMPLE 2

As in the embodiment 1, 10 batches were produced of spherical Er₃ Niparticles with diameters of between 0.2.sup.˜ 0.3 mm of which particleswith an aspect ratio of not more than 5 constituted not less than 90 wt.%. Next, 1 g of particles was randomly extracted from each of the tenbatches of spherical Er₃ Ni particles. These extracted particles wereeach filled within the die 2 for the mechanical strength evaluationshown in FIG. 1 and a compressive stress of 3 MPa (crosshead speed=0.1mm/min) was applied using an Instron-type testing machine, but hardlyany particles were fractured. Since hardly any particles are fracturedby a compressive stress of less than 5 MPa, reliability cannot beevaluated.

EMBODIMENT 3

First, an Er₃ Co mother alloy was prepared by high frequency fusion.This Er₃ Co mother alloy was melted at approximately 1373 K and the meltthereby obtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidified. The particlesobtained were sieved and classified according to form and 1 kg ofspherical particles with diameters of between 200.sup.˜ 300 μm wasselected. Particles with an aspect ratio of not more than 5 constitutednot less than 90 wt. % of all the particles. This process was carriedout repeatedly and 10 batches of spherical Er₃ Co particles wereobtained.

Next, 1 g of particles was randomly extracted from each of theabove-mentioned 10 batches of spherical Er₃ Co particles. Theseextracted particles were each filled within a die 2 for mechanicalstrength evaluation shown in FIG. 1 and a compressive stress of 5 MPa(crosshead speed=0.1 mm/min) was applied thereto using an Instron-typetesting machine. Following the test, all particles were sieved andclassified according to form and the weight of the fractured sphericalEr₃ Co particles was measured. The rates of particles fractured areshown in Table 2. When the form factor R of each of these magneticregenerator particles was evaluated by image analysis, all rates ofparticles in which R was more than 1.5 were not more than 5%.

The above-mentioned magnetic regenerator spherical particles of Er₃ Cowere filled in a regenerator at a packing factor of 70%, respectively,put in a two-stage GM refrigerator identical to that in the embodiment 1and refrigerator testing was carried out. Test results are also shown inTable 2.

                  TABLE 2                                                         ______________________________________                                               Rate of particles                                                             fractured by   refrigeration                                                  compressive    capacity (mW)                                                    stress test of   Initial                                                                              After 5000                                   Test No. 5 MPa (wt. %)    Value  hours                                        ______________________________________                                        Embodiment 3                                                                  1        0.002            306    305                                          2        0.003            309    308                                          3        0.109            302    297                                          4        0.021            305    302                                          5        0.007            308    308                                          6        0.030            302    299                                          7        0.004            306    304                                          8        0.005            300    298                                          9        0.043            306    303                                          10       0.007            309    309                                          ______________________________________                                    

As Table 2 clearly shows, all the regenerators using magneticregenerator particles in which the rate of particles fractured when acompressive stress of 5 MPa was applied was not more than 1 wt. %maintained excellent refrigeration capacity over a long period of time.

Furthermore, it was confirmed from this embodiment 3 and fromembodiments 1 and 2 described above that irrespective of the compositionand such like of the magnetic regenerator material, when magneticregenerator particles in which the rate of particles fractured when acompressive stress of 5 MPa was applied was not more than 1 wt. % areused, excellent refrigerating capability can be maintained over a longperiod of time.

EMBODIMENT 4, COMPARATIVE EXAMPLE 3

An ErAg mother alloy was prepared by high frequency fusion. This ErAgmother alloy was melted at approximately 1573 K and the melt therebyobtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidified. The particlesobtained were sieved and classified according to form and 1 kg ofspherical particles with diameters of between 0.2.sup.˜ 0.3 mm wasselected. Particles with an aspect ratio of not more than 5 constitutednot less than 90 wt. % of all the particles. This process was carriedout repeatedly and 5 batches of spherical ErAg particles were obtained.

Next, 1 g of particles was randomly extracted from each of theabove-mentioned 5 batches of spherical ErAg particles. These extractedparticles were each filled within a die 2 for mechanical strengthevaluation shown in FIG. 1 and a compressive stress of 5 MPa (crossheadspeed=0.1 mm/ml) was applied using an Instron-type testing machine.Following the test, all particles were sieved and classified accordingto form and the weight of the fractured spherical ErAg particles wasmeasured. The rates of particles fractured are shown in Table 3.

The above-mentioned magnetic regenerator spherical particles of ErAgwere filled in regenerator at a packing factor of 64%. Theseregenerators were then put in a two-stagte GM refrigerator as a secondregenerator respectively and refrigerator testing was carried out tomeasure the lowest temperatures attained by the refrigerators. Initialvalues of lowest temperatures attained and lowest temperatures achievedafter 5000 hours of continuous operation are shown respectively in Table3.

                  TABLE 3                                                         ______________________________________                                               Rate of particles                                                                           Lowest                                                          fractured by  Temperature                                                     compressive   Attained (K)                                                      stress test of  Initial                                                                              After 5000                                    Test No. 5 MPa (wt. %)   Value  hours                                         ______________________________________                                        Embodiment 4                                                                  1        0.031           6.3    7.6                                           2        0.003           6.7    7.4                                           3        0.107           6.6    8.3                                           Comparative Example 3                                                         4        1.259           6.7    15.4                                          5        2.117           6.5    23.8                                          ______________________________________                                    

EMBODIMENT 5, COMPARATIVE EXAMPLE 4

First, an ErNi mother alloy was prepared by high frequency fusion. ThisErNi mother alloy was melted at approximately 1473 K and the meltthereby obtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidified. The particlesobtained were sieved and classified according to form and 1 kg ofspherical particles with diameters of between 0.25.sup.˜ 0.35 mm wasselected. Particles with an aspect ratio of not more than 5 constitutednot less than 90 wt. % of all the particles. This process was carriedout repeatedly and 5 batches of spherical ErNi particles were produced.In addition, 5 batches of spherical Ho₂ Al particles were produced.

Next, 1 g of particles was randomly extracted from each of theabove-mentioned 5 batches of spherical ErNi particles and the 5 batchesof spherical Ho₂ Al particles. The extracted particles were each filledwithin a die 2 for mechanical strength evaluation shown in FIG. 1 and acompressive stress of 5 MPa (crosshead speed=0.1 mm/min) was appliedthereto using an Instron-type testing machine. Following the test, allparticles were sieved and classified according to form and the weight ofthe fractured particles was measured. The rates of particles fructuredare shown in Table 4.

The magnetic regenerator spherical particles of ErNi and Ho₂ Al werefilled in regenerator in a 2-layered structure in which ErNi particlesoccupied the lower temperature half side and Ho₂ Al particles occupiedin the higher temperature half side at a packing factor of 64%,respectively. Each of these regenerators was then put in a two-stage GMrefrigerator as second regenerators and refrigerator testing was carriedout to measure the lowest temperatures attained by the refrigerator.Initial values of lowest temperatures attained and lowest temperaturesachieved after 5000 hours of continuous operation are shown respectivelyin Table 4.

                  TABLE 4                                                         ______________________________________                                                   Rate of particles                                                                         Lowest                                                            fractured by                                                                              Temperature                                                       compressive Attained (k)                                                             stress test of                                                                             Initial                                                                            After 5000                                Test No.          5 MPa (wt. %)                                                                              Value                                                                              hours                                     ______________________________________                                        Embodiment 5                                                                  1        ErAg    0.003         3.4  3.7                                                Ho.sub.2 Al                                                                           0.005                                                        2        ErAg    0.005         3.6  4.1                                                Ho.sub.2 Al                                                                           0.048                                                        3        ErAg    0.016         3.4  3.9                                                Ho.sub.2 Al                                                                           0.009                                                        Comparative Example 4                                                         4        ErAg    1.600         3.7  7.3                                                Ho.sub.2 Al                                                                           1.233                                                        5        ErAg    1.706         3.9  8.3                                                Ho.sub.2 Al                                                                           1.727                                                        ______________________________________                                    

EMBODIMENT 6, COMPARATIVE EXAMPLE 5

An HoCu₂ mother alloy was prepared by high frequency fusion. This HoCu₂mother alloy was melted at approximately 1373 K and the melt therebyobtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kpa) and rapidly solidified. The particlesobtained were sieved to adjust diameters 0.2.sup.˜ 0.3 mm, shapeseparation was carried out using an inclined vibrating plate method and1 kg of spherical particles was selected. Particles with an aspect ratioof not more than 5 constituted not less than 90 wt. % of all theparticles. This process was carried out repeatedly and 5 batches ofspherical HoCu₂ particles were produced. The roundness of each batch ofspherical HoCu₂ particles was then altered by adjusting shape separationconditions such as for instance an angle of inclination and vibrationpower.

The perimeter of a projected image L and the area of the projected imageA of each particle of the 5 batches of spherical HoCu₂ particlesobtained were measured by image analysis and a form factor R expressedby L² /4πA was evaluated. Results are shown in Table 5.

In addition, 1 g of particles was randomly extracted from each of theabove-mentioned 5 batches of spherical HoCu₂ particles. These extractedparticles were each filled within a die 2 for mechanical strengthevaluation shown in FIG. 1 and a compressive stress of 5 MPa (crossheadspeed=0.1 mm/min) was applied thereto using an Instron-type testingmachine. Following the test, all particles were sieved and classifiedaccording to form and the weight of the fractured spherical HoCu₂particles was measured. The rates of particles fractured are shown inTable 5.

The magnetic regenerator spherical particles of HoCu₂ were filled inregenerator, respectively, at a packing factor of 64%. Theseregenerators were then put respectively in two-stage GM refrigerators assecond regenerator and refrigerator testing was carried out to measurethe lowest temperatures attained by the refrigerators. Initial values oflowest temperatures attained and lowest temperatures achieved after 5000hours of continuous operation are also shown respectively in Table 5.

                  TABLE 5                                                         ______________________________________                                        Rate of         Rate of particles                                                                         Lowest                                            particles       fractured by                                                                              Temperature                                       each of which   compressive Attained (K.)                                             R is more   stress test of                                                                            Initial                                                                             After 5000                              Test No.                                                                              than 1.5 (%)                                                                              5 MPa (wt. %)                                                                             Value hours                                   ______________________________________                                        Embodiment 6                                                                  1       0.6         0.012       5.1   5.6                                     2       1.5         0.007       5.3   5.9                                     3       6.6         0.040       5.5   6.6                                     4       5.6         0.307       6.7   8.2                                     Comparative Example 5                                                         5       7.9         1.474       6.5   13.8                                    ______________________________________                                    

EMBODIMENT 7

First, an Er₃ Ni mother alloy was prepared by high frequency fusion.This Er₃ Ni mother alloy was melted at approximately 1373 K and the meltthereby obtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidified. The particlesobtained were sieved and particles with diameters of 0.2.sup.˜ 0.3 mmwere obtained. Furthermore, shape separation using inclined vibratingplate method was carried out to the particles thereby obtained, toremove particles with high partial irregularity and to select Er₃ Nispherical particles with low partial irregularity.

The perimeter of a projected image L and the area of the projected imageA of each particle of obtained the Er₃ Ni spherical particles weremeasured by image analysis and a form factor R expressed by L² /4πA wasevaluated. The result showed that the rate of particles with a formfactor R more than 1.5 was 0.6% and that the rate of particles with aform factor R more than 1.3 was 4.7%. The aspect ratio for all particleswas not more than 5.

Magnetic regenerator spherical particles of Er₃ Ni selected by themethod described above were filled in a regenerator at a packing factorof 70%. This regenerator was then put in a two-stage GM refrigerator andrefrigerator testing was carried out. As a result, an initialrefrigeration capacity of 320 mW was obtained at 4.2 K and stablerefrigeration capacity was obtained over 5000 hours of continuousoperation.

EMBODIMENT 8

An Er₃ Ni mother alloy was prepared by high frequency fusion. This Er₃Ni mother alloy was melted at approximately 1300 K and the melt therebyobtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 30 kPa) and rapidly solidified. The particlesobtained were sieved and particles with diameters of 0.2.sup.˜ 0.3 mmwere obtained. Furthermore, shape separation using inclined vibratingplate method as in the embodiment 7 was carried out to the particlesthereby obtained, to remove particles with high partial irregularity andto select Er₃ Ni spherical particles with low partial irregularity.

The perimeter of a projected image L and the area of the projected imageA of each particle of the Er₃ Ni spherical particles obtained weremeasured by image analysis and a form factor R expressed by L² /4πA wasevaluated. The result showed that the rate of particles with a formfactor R more than 1.5 was 4% and the rate of particles with a formfactor R more than 1.3 was 13%. However, particles with an aspect ratiomore than 5 constituted 32 wt. % of all particles.

Magnetic regenerator spherical particles of Er₃ Ni selected by themethod described above were filled in a regenerator at a packing factorof 70%, placed in a two-stage GM refrigerator and refrigerator testingwas carried out. As a result, an initial refrigeration capacity of 310mW was obtained at 4.2 K and refrigeration capacity after 5000 hours ofcontinuous operation was 305 mW.

COMPARATIVE EXAMPLE 6

Shape separation of particles produced and sieved as in the embodiment 7was carried out using a inclined vibrating plate with a comparativelysmaller angle of inclination than in the embodiment 7 and Er₃ Nispherical particles were selected. When the aspect ratio of the Er₃ Nispherical particles obtained was measured, the aspect ratio of allparticles was not more than 5. Furthermore, evaluation of the formfactor R of the Er₃ Ni spherical particles as in the embodiment 7revealed that the rate of particles with a form factor R more than 1.5was 7% and the rate of particles with a form factor R more than 1.3 was24%.

The above-mentioned Er₃ Ni spherical particles were filled in aregenerator at a packing factor of 70%, placed in a two-stage GMrefrigerator and refrigerator testing was carried out. The result wasthat an initial refrigeration capacity of 320 mW was obtained at 4.2 Kbut after 5000 hours of continuous operation refrigeration capacity haddeteriorated to 280 mW.

COMPARATIVE EXAMPLE 7

An Er₃ Ni mother alloy was prepared by high frequency fusion. This Er₃Ni mother alloy was melted at approximately 1273 K and the melt therebyobtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidificated. Theparticles obtained were sieved and particles with diameters of 0.2.sup.˜0.3 mm were obtained. Furthermore, shape separation using inclinedvibrating plate method as in the Comparative Example 6 was carried outto the particles obtained and spherical particles were selected.

When the aspect ratio of the Er₃ Ni spherical particles obtained wasmeasured, particles with an aspect ratio more than 5 constituted 34 wt.% of all particles. In addition, when the form factor R of the Er₃ Nispherical particles was evaluated by the same method as in theembodiment 7, the rate of particles with a form factor R more than 1.5was 11% and the rate of particles with a form factor R more than 1.3 was27%.

The above-mentioned Er₃ Ni spherical particles were filled in aregenerator at a packing factor of 70%, placed in a two-stage GMrefrigerator and refrigerator testing was carried out. The result wasthat an initial refrigeration capacity of 320 mW was obtained at 4.2 Kbut after 5000 hours of continuous operation refrigeration capacity haddeteriorated to 270 mW.

EMBODIMENT 9

An Er₃ Co mother alloy was prepared by high frequency fusion. This Er₃Co mother alloy was melted at approximately 1373 K and the melt therebyobtained was poured onto a rotating disc in Ar atmosphere(pressure=approximately 101 kPa) and rapidly solidificated. Theparticles obtained were sieved and particles with diameters of 0.2.sup.˜0.3 mm were obtained. Furthermore, shape separation using inclinedvibrating plate method was carried out to the particles obtained, toremove particles with high partial irregularity and to select Er₃ Cospherical particles with low partial irregularity.

The perimeter of a projected image L and the area of the projected imageA of each particle of the Er₃ Co spherical particles obtained weremeasured by image analysis and a form factor R expressed by L² /4πA wasevaluated. The result showed that the rate of particles with a formfactor R more than 1.5 was 0.2% and the rate of particles with a formfactor R more than 1.3 was 3.3%. Furthermore, the aspect ratio of allparticles was not more than 5.

Magnetic regenerator spherical particles of Er₃ Co selected by themethod described above were filled in a regenerator at a packing factorof 70%, placed in a two-stage GM refrigerator and refrigerating testingwas carried out. As a result, an initial refrigeration capacity of 250mW was obtained at 4.2 K and stable refrigeration capacfity was obtainedover 5000 hours of continuous operation.

INDUSTRIAL APPLICABILITY

As the above embodiments clearly show, according to a regeneratormaterial for extremely low temperatures of the present invention,excellent mechanical properties for mechanical vibration can be obtainedwith a high reproducibility. Therefore, a regenerator for extremely lowtemperatures of the present invention using such regenerator material iscapable of maintaining excellent refrigerating performance for a longperiod of time with a high reproducibility.

What is claimed is:
 1. A regenerator material for extremely lowtemperatures comprising:magnetic regenerator particles, wherein when acompressive stress of 5 MPa is applied to the magnetic regeneratorparticles, the magnetic regenerator particles comprise 1 wt. % or lessof fractured magnetic regenerator particles.
 2. A regenerator materialfor extremely low temperatures according to claim 1, wherein:5% or lessof the magnetic regenerator particles have a form factor R of more than1.5, wherein R is expressed by L² /4πA, wherein L represents a perimeterof a projected image of each magnetic regenerator particle and Arepresents an area of the projected image.
 3. A regenerator material forextremely low temperatures according to claim 1, wherein:70 wt. % ormore of the magnetic regenerator particles have a ratio of the majordiameter to the minor diameter equal to or less than
 5. 4. A regeneratormaterial for extremely low temperatures according to claim 1, wherein:70wt. % or more of the magnetic regenerator particles have a diameter Dsatisfying the expression 0.01≦D≦3.0 mm.
 5. A regenerator material forextremely low temperatures according to claim 1 wherein:the magneticregenerator particles consist of intermetallic compounds including rareearth elements expressed by RM_(z), wherein R represents at least onerare earth element selected from the group consisting of Y, La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; M represents at least onemetallic element selected from the group consisting of Ni, Co, Cu, Ag,Al and Ru; and z represents a number satisfying the expression0.001≦z≦9.0 or intermetallic compounds including rare earth elementsexpressed by ARh, wherein A represents at least one rare earth elementselected from the group consisting of Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb.6. A regenerator material for extremely low temperaturescomprising:magnetic regenerator particles, wherein, 5% or less of themagnetic regenerator particles have a form factor R of more than 1.5,wherein R is expressed by L² /4πA, wherein L represents a perimeter of aprojected image of each magnetic regenerator particle and A representsan area of the projected image.
 7. A regenerator material for extremelylow temperatures according to claim 6, wherein,in the magneticregenerator particles, 70 wt. % or more of the magnetic regeneratorparticles have a ratio of the major diameter to the minor diameter equalto or less than
 5. 8. A regenerator material for extremely lowtemperatures according to claim 6, wherein:70 wt. % or more of themagnetic regenerator particles have a diameter D satisfying theexpression 0.01≦D≦3.0 mm.
 9. A regenerator material for extremely lowtemperatures according to claim 6, wherein:the magnetic regeneratorparticles consist of intermetallic compounds including rare earthelements expressed by RM_(z), wherein R represents at least one rareearth element selected from the group consisting of Y, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; M represents at least onemetallic element selected from the group consisting of Ni, Co, Cu, Ag,Al and Ru; and z represents a number satisfying the expression0.001≦z≦9.0 or intermetallic compounds including rare earth elementsexpressed by ARh, wherein A represents at least one rare earth elementselected from the group consisting of Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb.10. A regenerator for extremely low temperatures comprising:aregenerator container; and regenerator material for extremely lowtemperatures, the regenerator material comprising magnetic regeneratorparticles, which fill inside the regenerator container and when acompressive stress of 5 MPa is applied to the magnetic regeneratorparticles, the magnetic regenerator particles comprise 1 wt. % or lessof fractured magnetic regenerator particles.
 11. A regenerator forextremely low temperatures according to claim 10, wherein:5% or less ofthe magnetic regenerator particles have a form factor R of more than1.5, wherein R is expressed by L² /4πA, wherein L represents a perimeterof a projected image of each magnetic regenerator particle and Arepresents an area of the projected image.
 12. A regenerator forextremely low temperatures according to claim 10, wherein:70 wt. % ormore of the magnetic regenerator particles have a ratio of the majordiameter to the minor diameter equal to or less than
 5. 13. Aregenerator for extremely low temperatures according to claim 10,wherein:70 wt. % or more of the magnetic regenerator particles have adiameter D satisfying the expression 0.01≦D≦3.0 mm.
 14. A regeneratorfor extremely low temperatures according to claim 10, wherein:themagnetic regenerator particles consist of intermetallic compoundsincluding rare earth elements expressed by RM_(z), wherein R representsat least one rare earth element selected from the group consisting of Y,La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; M representsat least one metallic element selected from the group consisting of Ni,Co, Cu, Ag, Al and Ru; and z represents a number satisfying theexpression 0.001≦z≦9.0 or intermetallic compounds including rare earthelements express by ARh, wherein A represents at least one rare earthelement selected from the group consisting of Sm, Gd, Tb, Dy, Ho, Er, Tmand Yb.
 15. A regenerator for extremely low temperatures comprising:aregenerator container; and regenerator material for extremely lowtemperatures consisting of magnetic regenerator particles filled insidethe regenerator container, in which 5% or less of the magneticregenerator particles have a form factor R of more than 1.5, wherein Ris expressed by L² /4πA, wherein L represents a perimeter of a projectedimage of each magnetic regenerator particle and A represents an area ofthe projected image.
 16. A regenerator for extremely low temperaturesaccording to claim 15, wherein:70 wt. % or more of the magneticregenerator particles have a ratio of the major diameter to the minordiameter equal to or less than
 5. 17. A regenerator for extremely lowtemperatures according to claim 15 wherein:70 wt. % or more of themagnetic regenerator particles have a diameter D satisfying theexpression 0.01≦D≦3.0 mm.
 18. A regenerator for extremely lowtemperatures according to claim 15, wherein:the magnetic regeneratorparticles consist of intermetallic compounds including rare earthelements expressed by RM_(z), wherein R represents at least one rareearth element selected from the group consisting of Y, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; M represents at least onemetallic element selected from the group consisting of Ni, Co, Cu, Ag,Al and Ru; and z represents a number satisfying the expression0.001≦z≦9.0 or intermetallic compounds including rare earth elementsexpressed by ARh, wherein A represents at least one rare earth elementselected from the group consisting of Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb.19. A refrigerator comprising a regenerator for extremely lowtemperatures according to claim
 10. 20. A refrigerator comprising aregenerator for extremely low temperatures according to claim
 15. 21. Amanufacturing method of a regenerator material for extremely lowtemperatures comprising the steps of:providing magnetic regeneratorparticles, and testing the particles by applying a compressive stress of5 MPa to a representative sample of the particles, selecting themagnetic particles in which the representative sample of magneticregenerator particles comprise 1 wt % or less of fractured particles.22. A manufacturing method of a regenerator material for extremely lowtemperatures comprising the steps of:providing magnetic regeneratorparticles; testing the magnetic regenerator particles by applying acompressive stress of 5 MPa to a representative sample of particlesextracted from the magnetic regenerator particles, and selecting themagnetic regenerator particles in which the extracted sample of magneticregenerator particles comprise 1 wt % or less of fractured particles.23. A manufacturing method of a regenerator material for extremely lowtemperatures comprising:providing a plurality of batches of magneticregenerator particles; and testing each batch of magnetic regeneratorparticles by applying a compressive stress of 5 MPa to a representativesample of particles extracted from each batch, and selecting the batchesin which the representative sample particles of each batch comprises 1wt % or less of fractured particles.